Just to confirm my early production version of the ARC AIR included two of the single button versions of the wireless throttles. These have a small pcb which has two pcb buttons fitted i.e. one remains concealed. I can confirm this concealed button is FULLY FUNCTIONAL for lane changing with ARC PRO.
Hence, if you want to use with ARC PRO there are three simple options:
1/ cross wire two of the wires to this button pcb (yellow and white if I remember correctly) so you have lane change functionality in place of brake.
2/ use the slot-it 3-D printed plastic button top and cut the casing as appropriate.
3/ wait for someone to design a better 3-D replica of the standard button top.
Hope the above positive update is helpful.
Long shot but I don't suppose you have a pic of the wiring job for the controller to make it useable for Arc Pro ?